Radio frequency communication devices, such as two-way radios or cellular phones, are known to have circuitry to receive or transmit radio frequency signals via antennas. An example of such radio communication devices and their antenna is seen in U.S. Pat. No. 5,438,339 wherein a `miniature high performance antenna` is realized with a retractable first portion and a fixed external second portion operative `to promote efficient use of the limited space available in the casing`. Generally, these antennas are designed to feed desired radio frequency signals to the circuitry. The antenna is the critical element of a communication device. Its performance determines the overall efficiency of the device. The antenna is desirably small, at least during storage, in portable devices. An example of a retractable antenna is seen in U.S. Pat. No. 5,861,859 wherein is achieved an antenna assembly `which requires a smaller housing space`.
In the art, an antenna operating at a desired radio frequency signal is said to resonate at the frequency of that signal. Short antenna lengths are desirable when the device is to be hand held. A short antenna is typically brought to resonance by loading it with an inductor. A loading coil for an antenna is described in U.S. Pat. No. 4,161,710 wherein the coil is tapped to enable selection between frequencies. A smaller antenna may be effected with a one-quarter wavelength (1/4.lambda.) antenna which will resonate over a ground plane. The ground to the circuit board of the device can be used as the ground plane to the antenna.
There is a desire for smaller hand held communication devices. The printed circuit board to the devices has been made smaller and the available ground plane is consequently smaller.
There is a strong dependency of the antenna on the small ground plane which is effected within a hand held device such that further reduction of size of the communication device and consequent reduced circuit board size results in less effective ground plane to the antenna such that it is more easily de-tuned when a user holds the device. A consequence is reduced efficiency. The presence of the user results in the resonant frequency of the antenna shifting away from the desired frequency. This causes poor consistency and reliability in the performance of the device and a reduction in battery life. There is a need to alleviate the problem of antenna detuning in these communication devices, especially portable devices.
An antenna is an assembly that can include the housing of a radio communication device or some portion of that housing. For example, U.S. Pat. No. 4,491,843 describes a dipole antenna formed with a metal plate and a metal box that encloses radio circuitry and placed at a predetermined distance from the metal plate. In U.S. Pat. No. 5,940,039 is a hand held communications device in which a `shielding case` to a loading coil changes `the polarizing mode of the antenna` so that a user's head may be in a radiation safe area. Whilst such conductive elements of an area extended type are known, and antennas have been developed in a range of forms as described above, there remains a problem with holding a quarter wave antenna at a desired frequency.